The seeds of legumes (pulses) include beans, peas and lentils. Legumes show beneficial health effects when consumed by humans, and one way that legumes are thought to be beneficial to is through their beneficial glycaemic effects. In this regard legumes are able to favourably modulate blood sugar levels and this may explain their anti-obesity effects. Some studies using in vitro models of the gut have shown that the addition of legume fibre to a glucose solutions is able to reduce the emptying rate of a dialysis sac of glucose. Legumes contain a range of water soluble fibres that produce viscous solutions when mixed with water and these include guar gum, pectin and tragacanth. It has been suggested that the increase viscosity caused by these fibres is the reason for the delayed glucose emptying rate. Based on such viscosity experiments in is often suggested that the water soluble fibres form a physical barrier in the unstirred layer of the intestine and this may slow the absorption of glucose to the blood.
However, some evidence suggests that the beneficial effects of the legume fibre actually comes from its ability to sequester the starch content of the legumes away from the digestive processes, rather than through the formation of a viscous solution in the unstirred layer of the intestine. In this way the intact or partially intact cell wall may provide a physical barrier to hydrolysis enzymes. Studies have compared the relative importance of the viscosity effect of legumes with the sequestration effects and results show that perhaps the latter is more important with regard contributing to the beneficial glycaemic effects. For example, in one study1, researchers measured the rate of glucose release from a dialysis bag in the presence and absence of lentils. However, the presence of lentils did not slow the release of the glucose. In addition, agitation of the bag did not increase the rate of glucose release. This suggests that the viscosity of the solution was not the overriding effect with regard inhibiting the release of the glucose.
However, when the authors ground the lentils and destroyed their physical form (including the cell wall structure), the rate of starch digestion was greatly increased. This suggests that it is the physical barrier of the cell wall surrounding the starch that contributes most significantly to the beneficial glycaemic effects of legumes. In fact grinding the lentils increased the starch hydrolysis rate 5-fold, whereas cooking them before grinding increased the starch hydrolysis rate about 2.5-fold. Other studies support these results in that it is the presence of the cells walls in the original plant material that provides most of the beneficial glycaemic effects to grains and pulses. Processed fibre or supplemental fibre does not produce the same glycaemic effects because the physical structure of the original cell walls is not present. While fibre may have multiple mechanisms of actions, in terms of the glycaemic effects it appears that the presence of the cell wall is paramount.
Dr Robert Barrington’s Nutritional Recommendation: For dietary fibre to be effective at producing beneficial glycaemic effects the fibre must be in its original plant form. This means that the cells walls must be partially intact to inhibit access to the starch content by the digestive enzymes. Eating whole foods is therefore paramount in maintaining correct glycaemic function.